Cvt control system having variable power source speed

ABSTRACT

A powertrain control system is disclosed. The powertrain control system may have a power source and a CVT coupled to an output of the power source. The powertrain control system may further have a controller in communication with the power source and the CVT. The controller may have a map with a plurality of speed modes, and, for at least one of the plurality of speed modes, the controller may be configured to vary an actual power source speed based on at least one of a CVT output speed or a ground speed.

TECHNICAL FIELD

The present disclosure relates generally to a control system for amachine with a CVT transmission and, more particularly, to a controlsystem for varying a power source speed based on a CVT output speed.

BACKGROUND

Machines such as, for example, on-highway vocational vehicles,off-highway haul trucks, wheel loaders, motor graders, and other typesof heavy machinery are used for a variety of tasks. These machinesgenerally include a power source, which may embody, for example, anengine, such as a diesel engine, a gasoline engine, or a gaseousfuel-powered engine that provides the power required to complete thesetasks. The power produced by the power source may be transmitted througha transmission, such as, for example, a continuously variabletransmission (“CVT”), to one or more ground engaging devices in order topropel the machine.

Machine control systems are often used to coordinate and regulateoperation of the power source and CVT to improve the machine'sresponsiveness and efficiency. For example, while the machine istraveling the power source and CVT may have a range of speeds andtorques at which the power source and CVT experience substantiallystable and efficient operation. Operating outside of this range mayincrease fuel consumption and/or decrease responsiveness.

One method for controlling a power source and CVT is disclosed in U.S.Pat. No. 7,192,374 (the '374 patent) issued to Kuras et al. on Mar. 20,2007. The '374 patent discloses an engine underspeed control system thatadjusts the transmission ratio so that the engine is running at anoptimal speed condition (i.e., within a range of speeds where the engineis operating most efficiently). The control system of the '374 patentdiscloses an operator input that provides an input signal to acontroller. The operator input, for example, could be an acceleratorpedal that allows the operator to depress the pedal to request anincrease in machine output speed. The input signal may represent arequested speed, which the controller then converts into a motor speedcommand (the motor being a component of a CVT that is powered by anengine). The control system of the '374 patent prevents the motor speedcommand from exceeding an upper speed limit and from dropping below alower speed limit. These limits are calculated such that, as long as themotor speed command remains within the upper and lower speed limits, themotor torque command will stay within the torque capability of themotor. The motor torque limit at a particular motor speed can bedetermined from the torque-speed curves for the motor. The engineunderspeed control algorithm (implemented by the controller) will alsoreduce the motor speed command if the engine begins lugging (e.g., ifthe engine speed drops below a threshold value). The method of the '374patent thus enables the CVT to respond quickly to changes in the motorspeed command while preventing damage to the motor and transmission.

Although the machine of the '374 patent may help the motor remainresponsive while preventing potential damage to the motor andtransmission, it may not provide for efficient operation and control ofthe engine under all conditions. By only controlling the motor speed,the control system of the '374 patent may allow the engine to operate atan inefficient and/or unresponsive engine speed (i.e., either too low ortoo high) for the presently occurring transmission gear ratio, workimplement conditions, and load conditions.

The disclosed machine system is directed to overcoming one or more ofthe problems set forth above.

SUMMARY OF THE DISCLOSURE

In one aspect, the present disclosure is directed to a powertraincontrol system. The powertrain control system may include a power sourceand a CVT coupled to an output of the power source. The powertraincontrol system may further include a controller in communication withthe power source and the CVT. The controller may include a map with aplurality of speed modes, and, for at least one of the plurality ofspeed modes, the controller may be configured to vary an actual powersource speed based on at least one of a CVT output speed or a groundspeed.

In another aspect, the present disclosure is directed to a method ofmachine control. The method may include generating a rotational outputand directing the rotational output to drive a CVT. The method mayfurther include measuring a CVT output speed and varying an actual speedof the rotational output based on the measured CVT output speed. Varyingthe actual speed may occur when implementing at least one of a pluralityof speed modes.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic illustration of an exemplary disclosed machine;

FIG. 2 is a schematic and diagrammatic illustration of an exemplarydisclosed powertrain and control system that may be used with themachine of FIG. 1; and

FIG. 3 is a graph of an exemplary map for controlling the powertrain ofFIG. 2.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary machine 10. Machine 10 may be a mobilevehicle that performs some type of operation associated with an industrysuch as mining, construction, farming, transportation, or any otherindustry known in the art. For example, machine 10 may be an earthmoving machine, such as a wheel loader, an excavator, a backhoe, a motorgrader, or any other suitable earth moving machine known in the art.Alternatively, machine 10 may be a load carrying vehicle, a marinevessel, a passenger vehicle, or any other suitable operation-performingmachine. Machine 10 may include one or more traction devices 12, a workimplement 16, an operator station 18, and a powertrain 20.

Traction devices 12 may include one or more wheels located on each sideof machine 10 (only one side shown) configured to allow translationalmotion of machine 10. Alternatively, traction devices 12 may includetracks, belts, or other traction devices known in the art. Any oftraction devices 12 may be driven and/or steerable.

Work implement 16 may include any device used to perform a particulartask, such as, for example, a bucket, a blade, a shovel, a ripper, ahammer, a grappling device, or any other task-performing device known inthe art. One or more work implements 16 may be attachable to machine 10and controllable from operator station 18. Work implement 16 may beconnected to machine 10 via a direct pivot or a linkage system and maybe actuated via one or more hydraulic actuators, electric motors, or inany other appropriate manner. Work implement 16 may pivot, rotate,slide, swing, lift, or move relative to machine 10 in any manner knownin the art.

Operator station 18 may be a location from which an operator controlsmachine 10. Operator station 18 may be located onboard or offboard ofmachine 10 and may include an operator input device 22 (see FIG. 2) forcontrolling one or more components of powertrain 20. Operator inputdevice 22 may be located proximal an operator seat and may embody asingle or multi-axis joystick, a wheel, a knob, a push-pull device, abutton, a pedal, or any other input device known in the art. It iscontemplated that operator station 18 may include additional operatorinput devices, such as, for example, a steering device, a brakingdevice, a gear ratio selection device, and/or other operator inputdevices known in the art.

As shown in FIG. 2, powertrain 20 may include components that worktogether to propel machine 10. Specifically, powertrain 20 may include apower source 24 drivingly coupled to a continuously variabletransmission (“CVT”) 26. It is contemplated that powertrain 20 may alsoinclude a torque converter (not shown) to couple power source 24 and CVT26.

Power source 24 may provide power output for the operation of machine 10(referring to FIG. 1). Power source 24 may embody a combustion engine,such as a diesel engine, a gasoline engine, a gaseous fuel poweredengine (e.g., a natural gas engine), or any other type of combustionengine known in the art. Power source 24 may alternatively embody anon-combustion source of power, such as a fuel cell or a power storagedevice coupled with an electric motor. Power source 24 may provide arotational output to drive traction device 12 (see FIG. 1), therebypropelling machine 10. Power source 24 may also provide a rotationaloutput to power a hydraulic circuit 25 used for actuating work implement16.

CVT 26 may include multiple subcomponents (or power flow paths) thattransmit rotational power from an output 30 of power source 24 totraction device 12. The subcomponents may include, for example, amechanical transmission 27 and a hydrostatic transmission 28. It iscontemplated that mechanical transmission 27 and hydrostatictransmission 28 may act in parallel, as shown in FIG. 2, or in series.

Mechanical transmission 27 of CVT 26 may embody, for example, amulti-speed, bidirectional, mechanical transmission with a plurality offorward gear ratios, one or more reverse gear ratios, and one or moreclutches (not shown). Mechanical transmission 27 may selectively actuatethe clutches to engage predetermined combinations of gears (not shown)to produce a discrete output gear ratio. Mechanical transmission 27 maybe an automatic-type transmission, wherein shifting is based on a powersource speed, a maximum operator selected gear ratio, and a shift mapstored within a controller. Alternatively, mechanical transmission 27may be a manual transmission, wherein the engaged gear is manuallyselected by an operator.

Hydrostatic transmission 28 may include a pump 38 and a motor 40interconnected by way of a first fluid passageway 42 and a second fluidpassageway 44. Pump 38 may embody, for example, a variable displacementpump rotated by output 30 of power source 24 to pressurize fluid. Pump38 may direct the pressurized fluid through fluid passageways 42 or 44to motor 40, thus creating a subsequent rotation of motor 40. A “gearratio” or “effective gear ratio” of hydrostatic transmission 28 may bealtered by varying the displacement of pump 38. It is contemplated thatwithin the operational limits of pump 38, the fluid displacement of pump38 may be infinitely varied (i.e., any fluid displacement within theoperational limits of pump 38 may be achievable), thus creating aninfinite number of effective gear ratios. Hydrostatic transmission 28may alternatively embody an electric continuously variable transmission,a roller-based continuously variable transmission, or a pulley-basedcontinuously variable transmission.

The outputs of mechanical transmission 27 and hydrostatic transmission28 may be combined using one or more gear assemblies 32 (only one shownin FIG. 2) disposed between mechanical and hydrostatic transmission 27,28 and a mechanical output 36. Gear assemblies 32 may include, forexample, planetary gear assemblies. Each gear assembly 32 may have, forexample, a carrier 33, a ring gear 35, and an sun gear 37. Sun gear 37may be connected to mechanical output 36, which may be coupled totraction device 12. Mechanical transmission 27 may be connected tocarrier 33 and hydrostatic transmission may be connected to ring gear35. It is contemplated that a parallel configuration may alternativelybe created by locating either hydrostatic transmission or mechanicaltransmission 27 on an output side of gear assemblies 32 (i.e., coupledto mechanical output 36) and then connecting a path of output 30directly into gear assemblies 32 (e.g., if mechanical transmission 27 islocated on the output end of gear assemblies 32, a path of output 30 maybe connected to carrier 33).

A combined gear ratio may be achieved by varying the discrete gear ratioof mechanical transmission 27 and the effective gear ratio ofhydrostatic transmission 28, thus changing the input and output torqueand speed characteristics of CVT 26. For example, the speed at whichring gear 35 rotates relative to a ground, and the speed at whichcarrier 33 rotates relative to ring gear 35, may determine a rotationalspeed of sun gear 37.

A control system 21 may monitor and modify the performance of machine 10and its components. In particular, control system 21 may include a speedsensor 46 and a controller 48. Controller 48 may communicate with speedsensor 46 via a communication line 50, with power source 24 via acommunication line 52, with CVT 26 via a communication line 54, and withoperator input device 22 via a communication line 56. It is contemplatedthat controller 48 may also communicate (not shown) with hydrauliccircuit 25 and/or other components of machine 10.

Speed sensor 46 may be located to sense a rotational speed of mechanicaloutput 36 (i.e., the CVT output speed). Speed sensor 46 may embody, forexample, a magnetic pick up sensor, a rotary encoder, a tachometer, orany other type of sensor configured to produce a corresponding signal.Speed sensor 46 may be disposed proximal a shaft associated withmechanical output 36 or proximal any other component of machine 10 whoserotational speed is related to the CVT output speed (e.g., an axle, awheel, a gear).

Controller 48 may embody a single microprocessor or multiplemicroprocessors that include a means for controlling an operation ofmachine 10. Numerous commercially available microprocessors may beconfigured to perform the functions of controller 48, and it should beappreciated that controller 48 may readily embody a general machinemicroprocessor capable of controlling numerous machine functions.Controller 48 may include a memory, a secondary storage device, aprocessor, and any other components for running an application. Variousother circuits may be associated with controller 48, such as, forexample, power supply circuitry, signal conditioning circuitry, dataacquisition circuitry, signal output circuitry, signal amplificationcircuitry, and other types of circuitry known in the art.

Controller 48 may include one or more maps stored within an internalmemory of controller 48. Each of these maps may include a collection ofdata in the form of tables, graphs, and/or equations. As shown in FIG.3, controller 48 may include at least one map 58 usable for controllinga power source speed limit (i.e., maximum speed of the power sourcerotational output) as a function of the CVT output speed and/or machineground speed (CVT output speed and machine ground speed may both bemeasurable or calculable using the signal received from speed sensor 46,and CVT output speed may be readily converted to machine ground speed,or vice versa). It is contemplated that the actual (or current) powersource speed may be may be below, but not above the power source speedlimit. Map 58 may specify the power source speed limit for a pluralityof speed modes, such as, for example, a low-speed mode 60, a mid-speedmode 62, and a high-speed mode 64. The speed modes may be directlyrelated to machine ground speed and/or CVT output speed. For examplelow, mid and high-speed modes 60, 62, and 64 may relate to ground speedsof approximately 0-8, 8-25, and 19-40 kilometers per hour, respectively(the speed modes may also be expressed in terms of CVT output speed). Itis contemplated that when using, for example, low-speed mode 60 ormid-speed mode 62, controller 48 may allow modulation of the actualpower source speed up to, but not exceeding the power source speedlimit. It is further contemplated that in at least one speed mode, suchas, for example, high-speed mode 64, controller 48 may control theactual power source speed (or power source speed command) based on theCVT output speed. For example, controller 48 may give an engine speedcommand to force an actual power source speed to the power source speedlimit (operator no longer directly controls the actual power sourcespeed with operator input device 22). Thus, in high-speed mode 64,operator input device 22 may control an output torque of CVT 26. It iscontemplated that each speed range may be optimized to maximize theefficiency and responsiveness of machine 10.

In low-speed mode 60 and when machine ground speed is about 0 kph, thepower source speed limit may be set at a maximum rated power sourcespeed, such as, for example, approximately 1700 rpm. Maximizing thepower source speed limit at a machine ground speed of 0 kph may create apotential for increased hydraulic fluid flow in hydraulic circuit 25. Inlow-speed mode 60, a trend of the power source speed limit may generallydecrease as the machine ground speed increases until it reachesapproximately 1600 rpm at a speed of 8 kph. If the actual power sourcespeed is tracking the power source speed limit, it is contemplated thatcontroller 48 may increase the machine travel speed while the decreasingthe actual power source speed (for a single discrete gear ratio) bymodifying the effective gear ratio of hydrostatic transmission 28. Forexample, if the operator has fully actuated operator input device 22(e.g., pedal is completely depressed), thus causing the actual powersource speed to track the power source speed limit, controller 48 mayincrease the machine travel speed while simultaneously decreasing thepower source speed by changing the rotational speed of ring gear 35 (viapump 38) at a faster rate than the rotational speed of carrier gear 33is changing. Controller 48 may continuously adjust the effective gearratio of hydrostatic transmission 28 independently of the currentlyselected discrete gear ratio to create any combined gear ratio thatachieves a specified power source speed trajectory (e.g., increasing,decreasing, or constant power source speed as a function of machineground speed) or meets another predetermined control objective ofcontroller 48 (e.g., specified propulsion, specified torque, specifiedfuel efficiency, and/or specified power available for work implementoperation).

In mid-speed mode 62, the power source speed limit may be set at asubstantially constant level, such as, for example, approximately 1600rpm. The power source speed limit of 1600 rpm may help achievesingle-function work implement cycle times. The machine travel speed atwhich controller 48 switches from mid-speed mode 62 to high-speed mode40 (thus switching between the power source speed limits used in eachspeed mode) may depend on the acceleration of machine 10 and/or a degreeof operator input device actuation (e.g., amount of pedal depression).It is contemplated that the machine acceleration may be calculated fromspeed sensor measurements or other appropriate means.

For example, when machine 10 is experiencing light acceleration and/or asmall amount of operator input device actuation, controller 48 mayswitch from mid-speed mode 62 to high-speed mode 64 at a lower machineground speed (e.g., approximately 19 kph). Alternatively, when machine10 is experiencing high acceleration and/or a large amount of operatorinput device actuation, controller 48 may switch from mid-speed mode 62to high-speed mode 64 at a higher machine ground speed (e.g., ofapproximately 25 kph). This delay of switching from the power sourcespeed limit of mid-speed mode 62 to the power source speed limit ofhigh-speed mode 64 may allow increased fuel efficiency and machinepropulsion under the heavy acceleration conditions. The power sourcespeed limit of the section of mid-speed mode 62 used during highacceleration conditions (e.g., section from approximately 19-25 kph) mayincrease from approximately 1600 rpm at 19 kph to approximately 1700 rpmat 25 kph.

In high-speed mode 64, when machine 10 is experiencing lightacceleration and/or a small amount of operator input device actuation(e.g., the section from approximately 19-25 kph), the actual powersource speed may be set at a substantially constant level, such as, forexample, approximately 1300 rpm. In the upper section of high-speed mode64 (e.g., section from approximately 25-40 kph), the power source speedlimit may increase as a function of ground speed. The power source speedlimit may have an increasing trajectory to offset the losscharacteristics of CVT 26, which may increase as a function of machineground speed. It is contemplated that the power source speed limit mayincrease to a value of approximately 1700 rpm at 40 kph. It is furthercontemplated that the power source speed limit may continue to increaseat the same rate for speeds above 40 kph or, alternatively, may plateauat 1700 rpm. All increases and/or decreases of the power source speedlimit in low, mid, and high-speed modes 60, 62, and 64 may beapproximately linear or defined by any other appropriate trajectory.

Each speed range in map 58 may be related to one discrete gear ratio ofmechanical transmission 27. For example, low-speed mode 50 may relate toa first discrete gear ratio, mid-speed mode 62 may relate to a seconddiscrete gear ratio, and high-speed mode 64 may relate to a thirddiscrete gear ratio. It is contemplated controller 48 may switch betweenthe plurality of speed modes in map 58 based on the machine travelspeed, the currently selected discrete gear ratio, or both.

INDUSTRIAL APPLICABILITY

The disclosed control system may be applicable to any machine wherepower source control is desired. The disclosed control system may modifya power source speed limit as a function of ground speed to reduce fuelconsumption while maximizing machine propulsion. The operation ofcontrol system 21 will now be described.

In one example, an operator of machine 10 may actuate operator inputdevice 22, thus requesting machine motion. Operator input device 22 maysend the operator request via communication line 56 to controller 48 andcontroller 48 may convert the signal to a requested power source speedand power source torque. Controller 48 may then reference the requestedpower source speed with map 58 to ensure that the requested power sourcespeed does not exceed the power source speed limit. Controller 48 mayset the actual power source speed to the requested power source speed upto but not exceeding the power source speed limit of low-speed mode 60.

During machine operation, controller 48 may continuously communicatewith speed sensor 46 to determine the machine ground speed and/or CVToutput speed. When the machine ground speed reaches approximately 8 kph,controller 48 may switch from the power source speed limit of low-speedmode 60 to the power source speed limit of mid-speed mode 62. At thetransition from low-speed mode 60 to mid-speed mode 62, controller 48may also command a switch from the first discrete gear ratio to thesecond discrete gear ratio.

While implementing mid-speed mode 62, the operator may continue tofreely modulate the actual power source speed up to but not exceedingthe power source speed limit. When the machine ground speed ofapproximately 19 kph is reached, controller 48 may communicate withspeed sensor 46. Controller 48 may use information from speed sensor 46to determine if machine 10 is accelerating. Controller 48 may alsocommunicate with operator input device 22 to determine the degree ofoperator input device actuation. If machine 10 is above a thresholdacceleration and/or operator input device 22 is above a threshold amountof actuation, controller 48 may continue to use the power source speedlimit of mid-speed mode 62 until a higher machine ground speed isreached, such as, for example, 25 kph. Alternatively, if machine 10 isbelow a threshold acceleration and/or operator input device 22 is belowa threshold amount of actuation, controller 48 may switch to the powersource speed limit of high-speed mode 64 at approximately 19 kph. It iscontemplated that at the transition from mid-speed mode 62 to high-speedmode 64, controller 48 may command a switch from the second discretegear ratio to the third discrete gear ratio.

While implementing high-speed mode 64, controller 48 may force an actualpower source speed to the power source speed limit, such that operatorinput device 22 may only control the output torque of CVT 26. It iscontemplated, however, that the entire power source speed limit curvemay scale up or down (e.g., entire curve on FIG. 3 that represents thepower source speed limit of high-speed mode 64 may move up or down)depending on a load carried or experienced by machine 10. For example,an increase in machine load (e.g., caused by usage of work implement 16)may cause the entire power source speed limit curve to scale up, and adecreased load may cause it to scale down. Controller 48 may stay inhigh-speed mode 64 until the machine ground speed decreases to theground speed ranges associated with either mid-speed mode 62, orlow-speed mode 60.

Several advantages of the disclosed control system may be realized. Inparticular, the disclosed control system may increase productivity andresponsiveness by allowing the operator to control the actual powersource speed at lower ground speeds to allow for increased flow for thepump associated with the work implement. However, the controller mayforce the actual power source speed to the power source speed limit athigher speeds to ensure maximum efficiency and propulsion. Additionally,the power source speed limit may vary as a function of machine groundspeed or CVT output speed to accommodate for the particular losscharacteristics of the power source and CVT combination.

It will be apparent to those skilled in the art that variousmodifications and variations can be made to the disclosed control systemwithout departing from the scope of the invention. Other embodiments ofthe control system will be apparent to those skilled in the art fromconsideration of the specification and practice of the control systemdisclosed herein. For example, any alternative CVT may be used in placeof the disclosed CVT, and all speed ranges may be related to virtualdiscrete gear ratios. It is intended that the specification and examplesbe considered as exemplary only, with a true scope being indicated bythe following claims and their equivalents.

1-27. (canceled)
 28. A method of machine control, comprising: operatinga power source to generate a power source rotational output; directingthe power source rotational output to drive a continuously variabletransmission; retrieving from a data map a plurality of speed modes ofcontrol of the power source rotational output, wherein the plurality ofspeed modes of control include a first speed mode and a second speedmode; retrieving from the data map a power source rotational outputspeed limit for each of the plurality of speed modes of control of thepower source rotational output, wherein the power source rotationaloutput speed limit of the first speed mode varies as a function of aground speed for a constant machine load, and wherein the power sourcerotational output speed limit of the first speed mode for a constantmachine load is a first value at zero ground speed, and a second valueat a ground speed greater than zero, wherein the first value is greaterthan the second value; transitioning from the first speed mode to thesecond speed mode as a result of a ground speed exceeding a thresholdspeed; and allowing direct modulation by an operator input device of anactual power source rotational output speed for the first speed mode upto but not above the power source rotational output speed limit of thefirst speed mode.
 29. The method of claim 28, further including scalingthe power source rotational output speed limit of the first speed modeup based on an increase in a machine load.
 30. The method of claim 29,further including retrieving from the data map a third speed mode ofcontrol of the power source rotational output and a power sourcerotational output speed limit for the third speed mode, wherein thepower source rotational output speed limit for the third speed modeincreases as a function of increasing ground speed at a constant machineload, and a power source rotational output speed limit of the secondspeed mode stays constant for increasing ground speed at a constantmachine load.
 31. The method of claim 30, further including varying theactual power source rotational output directly as a function of a groundspeed of the machine when implementing the third speed mode.
 32. Themethod of claim 31, further including controlling an output torque ofthe continuously variable transmission with the operator input devicewhen implementing the third speed mode.
 33. The method of claim 32,further including transitioning from the second speed mode to the thirdspeed mode as a result of the ground speed exceeding a second thresholdspeed, wherein the second threshold speed is set at one of a lowerthreshold speed or an upper threshold speed as a function of at leastone of a machine acceleration or a degree of operator input deviceactuation.
 34. The method of claim 28, further including measuring anoutput speed of the continuously variable transmission, and calculatingthe ground speed based on the measure output speed of the continuouslyvariable transmission.
 35. A method of machine control, comprising:operating a power source to generate a power source rotational output;directing the power source rotational output to drive a continuouslyvariable transmission, the continuously variable transmissiontransmitting rotational output of the power source to a traction device;measuring a rotational output speed of the continuously variabletransmission; retrieving from a data map a plurality of speed modes ofcontrol of the power source rotational output, wherein the plurality ofspeed modes of control include a first speed mode, a second speed mode,and a third speed mode; retrieving from the data map a power sourcerotational output speed limit for each of the plurality of speed modesof control of the power source rotational output, wherein the powersource rotational output speed limit of the first speed mode varies as afunction of the rotational output speed of the continuously variabletransmission for a constant machine load, and wherein the power sourcerotational output speed limit of the first speed mode for a constantmachine load is a first value at zero rotational output speed of thecontinuously variable transmission, and a second value at a rotationaloutput speed of the continuously variable transmission that is greaterthan zero, wherein the first value is greater than the second value;transitioning from the first speed mode to the second speed mode as aresult of a rotational output speed of the continuously variabletransmission exceeding a threshold speed; and increasing an actual powersource rotational output speed directly as a function of an increase inthe rotational output speed of the continuously variable transmissionduring the third speed mode.
 36. The method of claim 35, furtherincluding scaling the power source rotational output speed limit of thefirst speed mode up based on an increase in a machine load.
 37. Themethod of claim 35, further including allowing direct modulation by anoperator input device of an actual power source rotational output speedfor the first speed mode up to but not above the power source rotationaloutput speed limit of the first speed mode.
 38. The method of claim 35,further including scaling the power source rotational output speed limitof the first speed mode down based on a decrease in a machine load. 39.The method of claim 35, wherein the first speed mode is a low-speedmode, the second speed mode is a mid-speed mode, and the third speedmode is a high-speed mode, the power source speed limit of the low speedmode decreasing as a function of ground speed, wherein the power sourcespeed limit of the mid-speed mode is constant as a function of groundspeed.